Polyolefins and method for the production thereof

ABSTRACT

Process for producing polyolefins, comprising the following steps: 
     a) grafting of acid groups onto polyolefins by means of a graftable monomer bearing at least one functional group chosen from a carbonyl and an acid anhydride, optionally in the presence of another graftable monomer bearing a vinyl unsaturated group and, optionally, one or more aromatic rings; 
     b) purification, consisting in removing at least some of the graftable monomer bearing at least one functional group chosen from a carbonyl and an acid anhydride that has not reacted with the polyolefins; 
     c) neutralization of the acid groups by at least one neutralizing agent. 
     Polyolefin obtained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for producing polyolefinshaving special rheological and compatibilization properties, as well asto the resulting polyolefins and to their use.

The problems that arise with polymers in general, and polyolefins inparticular, relate to their insufficient melt strength when they arebeing processed by extrusion.

It is well known that the melt strength of polyethylene (PE) and ofpolypropylene (PP), which is defined by a high elongational viscosity,is insufficient in certain types of processing, such as extrusionfoaming, extrusion blow-moulding, thermoforming and blow moulding,particularly 3D blow moulding.

Solutions proposed for solving this problem consist in making themacromolecular structure of the PE or PP branched by creating covalentbonds between the macromolecules. However, in practice, branched resinsmanufactured by covalent coupling all suffer from a tendency of thebranches to degrade due to the effect of shear inherent in theprocessing. Moreover, significant irreversible covalent branchingresults in melt fractures, limiting the productivity and/or quality ofthe finished product.

In order to be able to increase the connection density betweenmacromolecules without being limited by crosslinking, it is possible toprovide a substantial part of the branches via reversible ionic bonds.This makes it possible to increase the melt strength while retaining thethermoplastic character

2. Description of the Related Art

The process disclosed in patent EP 0,086,159 proposes the crosslinkingof α-olefin polymers and copolymers in order to improve their meltstrength characteristics. The grafting of a carboxylic acid in thepresence of a radical generator and the subsequent salt formation bymetal compounds are envisaged.

BRIEF DESCRIPTION OF THE INVENTION

The objetive of the present invention is to provide a process for theproduction of polyolefins having improved properties, especially withregard to melt strength.

The present invention consequently relates to a process for producingpolyolefins, comprising the following steps:

a) grafting of acid groups onto polyolefins by. means of a graftablemonomer bearing at least one functional group chosen from a carbonyl andan acid anhydride, optionally in the presence of another graftablemonomer bearing a vinyl unsaturated group and, optionally, one or morearomatic rings;

b) purification, consisting in removing at least some of the grantablemonomer bearing at least one functional group chosen from a carbonyl andan acid anhydride that has not reacted with the polyolefins;

c) neutralization of the acid groups by at least one neutralizing agent.

The polymers obtained by the process according to the present inventionhave improved properties, especially with regard to melt strength, byvirtue of the introduction of an additional purification step b). Thisis because in the case of processes of the prior art, a relatively largeproportion of functional graftable monomers remains in the finalproduct, these monomers not having reacted with the polyolefins. Thepresence of these functional graftable monomers which have not reactedwith the polyolefins, that is to say which have not been grafted ontothe polyolefins after step a), may be responsible inter alia forinsufficient melt strength and for colour problems. The objective ofthis purification step b) is consequently to remove at least some of thefunctional graftable monomers that have not reacted with thepolyolefins.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the variation of elongational melt viscosity with timefor the polymer of the example.

FIG. 2 depicts the variation of the dynamic viscosity as a function offrequency.

FIG. 3 is a transmission electron microscope photograph.

The coordinate values in FIGS. 1 and 2 are marked according to thefollowing rule.

The symbol 1.E+01 represents 10¹=10, 1.E+02 represents 10²=100.

One advantageous embodiment of the present invention provides forpurification step b) to be carried out by one of the known, standardmethods, preferably by removal using acetone, by hot-air stripping, bysteam stripping, by stripping with an inert gas or by venting.

The grafting of the acid functional groups onto polyolefins is carriedout, for example, by a radical

DETAILED DESCRIPTION OF THE INVENTION

The polyolefins that can be used in the process according to theinvention are polymers of linear olefins containing from 2 to 8 carbonatoms, such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene and1-octene. The linear olefins preferably contain from 2 to 6 carbonatoms, more particularly from 2 to 4 carbon atoms.

The polyolefins may be selected from homopolymers of the aforementionedolefins and from copolymers of these olefins, particularly copolymers ofethylene or propylene with one or more comonomers. Advantageously, thecomonomers are chosen from the olefins described above and fromdiolefins containing from 4 to 18 carbon atoms, such as4-vinylcyclohexene, dicyclopentadiene, methylene- andethylidene-norbornene, 1,3-butadiene, isoprene and 1,3-pentadiene.

Preferably, the polyolefins are chosen from propylene polymers andethylene polymers. Most particularly preferred are the polyolefinschosen from ethylene homopolymer, propylene homopolymer, ethylenecopolymers, propylene copolymers, ethylene-propylene copolymers andblends thereof.

The propylene polymers are usually chosen from propylene homopolymersand copolymers whose melt flow index (MFI), measured at 230° C. andunder a load of 2.16 kg according to the ASTM D 1238 (1986) standard, isbetween 0.1 and 2000 dg/min., preferably between 0.1 and 500 dg/min. andparticularly preferably between 0.1 and 50 dg/min.

The ethylene polymers are usually chosen from ethylene homopolymers andcopolymers having a standard density of between 860 and 996 kg/m³,preferably between 915 and 960 kg/m³ and particularly preferably between936 and 953 kg/m³, and a melt flow index (measured at 190° C. under aload of 5 kg according to the ISO 1133 (1991) standard) of between 0.01and 2000 dg/min., preferably between 0.1 and 200 dg/min. andparticularly preferably between 1 and 40 dg/min.

The propylene polymers are most particularly preferred.

The graftable monomer bearing at least one functional group chosen froma carbonyl and an acid anhydride may be chosen, for example, fromunsaturated monocarboxylic or dicarboxylic acids and derivatives thereofand unsaturated monocarboxylic or dicarboxylic acid anhydrides andderivatives thereof. The graftable monomer preferably contains from 3 to20 carbon atoms. As typical examples, mention may be made of acrylicacid, methacrylic acid, maleic acid, fumaric acid, itaconic acid,crotonic acid, citraconic acid, maleic anhydride, itaconic anhydride,crotonic anhydride and citraconic anhydride. Maleic anhydride is mostparticularly preferred.

The amount of graftable monomer bearing at least one functional groupchosen from a carbonyl and an acid anhydride, used for the grafting,depends inter alia on the properties that it is intended to obtain inthe product, on the amount of radical generator used and therefore onthe grafting efficiency and the desired degree of grafting, as well ason the reaction time. In general, the amount of monomer is sufficient toimprove the properties of the end product and will generally be between0.01 and 10% by weight, preferably between 0.1 and 5% by weight, withrespect to the polyolefins.

The graftable monomer bearing a vinyl unsaturated group and, optionally,one or more aromatic rings preferably contains from 3 to 20 carbonatoms. As typical examples, mention may be made of 1-dodecene, styrene,vinylpyridine, divinylbenzene, 1,4-hexadiene and mixtures thereof.Styrene is most particularly preferred.

The use of this graftable monomer bearing a vinyl unsaturated group and,optionally, one or more aromatic rings makes it possible, especially insome cases, to increase the degree of grafting of the polyolefins.

If the graftable monomer bearing a vinyl unsaturated group and,optionally, one or more aromatic rings is used, the necessary proportionof this monomer depends on the intended properties and is in general0.01 to 10% by weight, preferably 0.1 to 5% by weight, with respect tothe polyolefins.

One particularly preferred form of the process of the present inventionprovides for this process to be carried out in the absence of anygraftable monomer bearing a vinyl unsaturated group and, optionally, oneor more aromatic rings.

The subject of another embodiment of the present invention is thereforea process in which the grafting of acid groups is carried out in thepresence of a radical generator.

As radical generators, an organic peroxide, and more particularly analkyl peroxide, are preferably used. Among these, mention may be made oftert-butylcumyl peroxide, 1,3-di(2-tert-butylper-oxyisopropyl)benzene,2,5-dimethyl-2,5-di(tert-butyl-peroxy)hexane, di(tert-butyl)peroxide and2,5-dimethyl2,5-di(tert-butylperoxy)-3-hexane. Particularly preferred is2,5-dimethyl-2,5-di-tert-butylperoxyhexane (DHBP).

In general, the radical generator is used in the process according tothe invention in an amount sufficient to allow the grafting to becarried out. Moreover, it is desirable for the amount not to exceed theminimum amount needed, since any excess radical generator may result indegradation of the polyolefin. The amount is usually at least equal to0.0005% by weight with respect to the polyolefins and is in particularat least equal to 0.001% by weight, values of at least 0.005% by weightbeing the most advantageous. In general, the amount does not exceed 3%tby weight, and preferably does not exceed 0.5% by weight, with respectto the polyolefins, values of at most 0.15 being the most recommended.

The step of neutralizing the acid groups is preferably carried out by atleast one neutralizing agent which includes a cationic part comprisingone or more cations chosen from the group consisting of alkali cations,alkaline-earth cations and transition-metal cations such as, forexample, Zn²⁺, Al³⁺ and Zr⁴⁺. Among alkali cations, Na⁺ is particularlypreferred. Among alkaline-earth cations, Mg²⁺ is particularly preferred.

The neutralizing agent preferably includes an anionic part comprisingone or more anions chosen from the group consisting of alcoholates,carboxylates, hydroxides, oxides, alkyls, carbonates andhydrogen-carbonates.

Examples of neutralizing agents are sodium hydroxide, calcium oxide,sodium carbonate, sodium hydrogencarbonate, sodium methoxide, sodiumacetate, magnesium ethoxide, zinc acetate, diethylzinc, aluminiumbutoxide, zirconium butoxide and similar compounds. Sodium hydroxide andzinc acetate are particularly preferred.

The amount of neutralizing agent added depends on the intended use andtherefore on the desired properties of the polyolefins. The neutralizingagent is used in an amount of 10% to 300% of the stoichiometric valuewith respect to the acid groups, preferably in an amount close tostoichiometry. In practice, the amount of neutralizing agent added willbe between 0.1 and 1000 meq/kg of polyolefin grafted with acid groups,preferably between 1 and 450 meq/kg of grafted polyolefin, more thanpreferably between 5 and 300 meq/kg and very particularly preferablybetween 10 and 100 meq/kg, depending on the degree of grafting of thepolyolefin.

The properties of the end product may be further improved by making stepc) be followed by a purification step d) consisting in removing theproducts arising from the reaction of neutralizing the grafted acidfunctional groups. This purification step may be carried out by any ofthe known, standard methods, preferably by steam stripping, by hot-airstripping or by venting, preferably by vacuum venting. This purificationis often indispensable since it makes it possible to shift theneutralization reaction equilibrium.

In order to carry out the grafting of the acid groups and, optionally,the following steps, all of the devices known for this purpose may beused. Thus, it is possible to work either with external mixers or withinternal mixers. Internal mixers are the more appropriate, and amongthese are BRABENDER® batch mixers, and continuous mixers such asextruders. An extruder in the context of the present invention comprisesat least the following parts: a feed zone and thereafter a dischargezone preceded by a compression zone, the latter forcing the melt to passthrough the discharge zone.

Reactive extrusion is a known process used for grafting functionalgroups and consequently, in a preferred embodiment of the processaccording to the present invention, the grafting step is carried out inan extruder, a technique generally called “reactive extrusion grafting”or “reactive extrusion”. Preferably, steps a) and c) of the process arecarried out in an extruder.

A particularly preferred extruder for carrying out the process accordingto the invention is made of a corrosion-resistant alloy. A particularlypreferred alloy is an alloy consisting mostly of nickel or cobalt.

Particularly preferably, all the steps are carried out in a continuousprocess comprising a single step for melting the polyolefins and morethan preferably all the steps may be carried out in a single extrusionin an extruder which generally comprises, in addition to theabovementioned zones, optionally, one or more staged feed devices forthe separate introduction of the polyolefin or polyolefins, of thegraftable monomers bearing at least one functional group chosen from acarbonyl and an acid anhydride, of the radical generator and/or of thestabilizer, one or more screw segments allowing propagation of thematerial to be extruded, one or more heating zones allowing theconstituents to be melted and, where appropriate, one or more ventingzones for the purification step or steps. These venting zones must beisolated from the reactant injection zones by a plug of molten material,generally achieved via pairs of screw segments skewed with respect tothe direction of flow. The discharge zone may furthermore be followed bya granulator or by a device giving the extrudate a profiled shape, suchas a film, a pipe, a sheet, etc. or with a device giving the extrudate afoamed profiled shape by the addition of a blowing agent.

The blowing agent used for the purposes of the present invention ischosen from the blowing agents normally used for generating cells inplastics, such as those described in the work entitled “Encyclopedia ofPolymer Science and Engineering, 2nd edition, Vol. 2, 1985, pp. 434-446.The blowing agent may be a blowing agent of the chemical type or of thephysical type. Preferably, the blowing agent is of the physical typesuch as, for example, an alkane (butane, propane, isobutane or pentane),a hydrofluorocarbon, carbon dioxide or mixtures thereof.

In practice, a particularly preferred process according to the presentinvention may comprise the following steps:

i) introduction and melting of the polyolefin(s);

ii) introduction of the graftable monomers bearing at least onefunctional group chosen from a carbonyl and an acid anhydride, of theradical generator and, optionally, of the graftable monomer bearing avinyl unsaturated group and, optionally, one or more aromatic rings;

iii) grafting of the graftable monomers bearing at least one functionalgroup chosen from a carbonyl and an acid anhydride;

iv) removal of the excess graftable monomers bearing at least onefunctional group chosen from a carbonyl and an acid anhydride that havenot reacted;

v) optional introduction of a stabilizer;

vi) introduction of the neutralizing agent;

vii) removal of the anion coming from the neutralizing agent; and

viii) granulation or extrusion of a profiled, optionally foamed, shape.

The temperature of the process is greater than the melting point andless than the decomposition temperature of the polyolefin and of thegrafted polyolefin, if necessary, where possible, at an optimumtemperature for the radical generator. The temperature therefore dependson the nature of the constituents of the reaction mixture and will ingeneral be at least 100° C., usually at least 130° C. and in particularat least 140° C. Generally, the process is carried out at a temperaturenot exceeding 400° C., usually not exceeding 300° C. and moreparticularly not exceeding 250° C.

The time needed to carry out the various steps of the process accordingto the present invention, in this case the grafting and/or priorpurification, the neutralization and/or the final purification, dependson the amounts of reactants used, on the temperature, on the nature ofthe reactants employed and on the type of reactor used (for example onthe type of extruder). In general, it is from 1 second to 1 hour,preferably from 5 seconds to 30 minutes and more particularly from 10seconds to 10 minutes.

During the process, it is possible to incorporate at any time one ormore standard additives for polyolefins, such as, for example,stabilizers, antioxidants, antistatic agents, organic dyes or mineralcolorants, and fillers, etc., as long as they do not interfere with thegrafting of the acid groups.

In a preferred form of the process according to the invention, at leastone stabilizer is added during the process.

Preferably, the stabilizer used in the process of the present inventionis chosen from the compounds comprising a sterically hindered phenolgroup, from phosphorus compounds and from mixtures thereof. These are,for example, substances such as1,3,5-trimethyl-2,4,6-tris(3,5-tert-butyl-4-hydroxybenzyl)benzene,pentaerythrityl tetrakis(3,5-di-tert-butyl-4-hydroxyphenylpropionate),tris(2,4-di-tert-butylphenyl)phosphite or a mixture of pentaerythrityltetrakis(3,5-di-tert-butyl-4-hydroxyphenylpropionate) andtris-(2,4-di-tert-butylphenyl)phosphite, preferably in equal amounts.The preferred stabilizer is1,3,5-trimethyl-2,4,6-tris(3,5-tert-butyl-4-hydroxybenzyl)benzene.

The invention also relates to the polyolefins resulting from the processaccording to the invention.

The invention furthermore relates to polyolefins containing partiallyneutralized acid groups, having a melt flow index of 0.001 to 1000dg/min. and an improved melt strength characterized by an exponentialincrease in the elongational viscosity and by an increase in the dynamicviscosity at low shear frequencies.

The polyolefins are those defined above.

The melt flow index of the polyolefins containing partially neutralizedacid groups is usually from 0.001 to 1000 dg/min., preferably between0.01 and 100 dg/min. and particularly preferably between 0.1 and 50dg/min., the melt flow index being measured, in the case of propylenepolymers, at 230° C. under a weight of 2.16 kg according to the ASTM D1238 (1986) standard ard, in the case of ethylene polymers, 190° C.under a weight of 5 kg according to the ISO 1133 (1991) standard.

Preferably, the polyolefins according to the invention are characterizedby ionic aggregates having a shape similar to that of a bunch of grapes,the size of the “bunch” being between 10 and 500 nm and the size of the“grapes” being less than 50 nm.

The “bunches of grapes” usually have a size of greater than 10 nm andpreferably greater than 50 nm.

The “bunches of grapes” usually have a size of less than 500 nm,preferably less than 200 nm.

The “grapes” constituting the “bunch” usually have a size of less than50 nm, preferably less than 25 nm and particularly preferably less than10 nm.

One useful application of the polyolefins is in the production of foams,especially high-density polyethylene foams and polypropylene foamsmanufactured by extrusion foaming. In particular, a useful applicationof the polyolefins is in the production of objects shaped by extrusionfoaming, thermoforming or blow moulding, particularly by 3D blowmoulding. another field of application is in improving the adhesion incompatibilization, multilayer and sealing applications.

The invention also relates to the extrusion foaming process in which theextrusion foaming is carried out consecutively to the polyolefinproduction process.

The term “consecutively” should be understood to mean that there is onlya single melting of the polyolefins for the polyolefin productionprocess and the extrusion foaming process.

The following example serves to illustrate the present invention withoutin any way limiting its scope.

EXAMPLE

The resin used is an ethylene-propylene random copolymer sold under thetrademark ELTEX® P KS 001 PF and is characterized by:

an MFI (melt flow index measured according to the ASTM D 1238 (1986)standard at 230° C. under a load of 2.16 kg) of 4.5 dg/min.;

a melting point of 134° C. (measured using the technique of DSC(Differential Scanning Calorimetry) according to the ISO FDIS 11357-3(1999) standard, at the second pass and with a scan rate of 10 K/min.);and

a total C2 content (measured by infrared spectrometry) of 4.6%.

The resin is fed into the extruder at a rate of 5 kg/h.

The extruder is a CLEXTRAL BC21 corotating twin-screw extruder. Thediameter of the screws is 25 mm and their length is 1200 mm. The speedof rotation of the screws is 300 rpm (rotations per minute). The barrelconsists of 12 barrel segments (zones) each having a separatetemperature control.

The 12 zones are respectively:

1. feed zone, the resin being fed via a hopper at a rate of 5 kg/h andthe temperature of the zone being 80° C.;

2. premelting compression zone (temperature: 180° C.);

3. mixing zone for the melting (temperature:

200° C.);

4. injection zone for injecting the acetone solution of maleic anhydrideand of the peroxide (temperature: 200° C.). The acetone solution ofmaleic anhydride, with a concentration of 187.5 g/litre, is introducedat a rate of 200 ml/h. The acetone solution of the peroxide DHPP, with aconcentration of 30 g/litre, is introduced at a rate of 100 ml/h. Thiszone is relatively sealed off upstream by a counterflight segment anddownstream by a grooved counterflight segment. These counterflightsreduce the rate of advance of the melt and create a dynamic plug;

5. venting zone for venting the unconverted reactants in zone 4 and theacetone (temperature: 240° C.);

6. injection zone for injecting the acetone solution of the1,3,5-trimethyl-2,4,6-tris(3,5-tert-butyl-4-hydroxybenzyl)benzenestabilizer having a concentration of 75 g/litre. The solution isintroduced at a rate of 200 ml/h. The temperature of the zone is 240°C.;

7. additional venting zone for venting the unconverted reactants in zone4 and acetone (temperature: 240° C.);

8. injection zone for injecting zinc acetate in aqueous solution with aconcentration of 220 g of zinc acetate per litre of aqueous solution.The rate of introduction is 200 ml/h and the temperature of the zone is180° C.;

9. venting zone for removing water and acetic acid (temperature: 240°C.);

10. mixing zone (temperature: 240° C.);

11. additional venting zone for removing water and acetic acid(temperature: 240° C.);

12. compression zone for forcing the material through the die(temperature: 200° C.).

After these 12 barrel zones, a die is used to convert the melt into arod which is cooled and converted into granules.

The final polymer is characterized by an MFI of 7.4 dg/min. It is alsocharacterized by the techniques of RME, ARES and transmission electronmicroscopy, as indicated below.

The elongational viscosity of the polymer in question is determined bymeans of a rheometer sold by Rheometrics under the name RME (Rheometricselongational rheometer for melts). The specimen (55×9×2 mm) is obtainedby extrusion and is subjected to a relaxation procedure before themeasurements. The curve plotted in FIG. 1 (RME plot) represents thevariation at 190° C. in the elongational melt viscosity (expressed inkpa.s) as a function of time (expressed in s) for a strain rate(expressed in s⁻¹) of 1.

The polymer produced in the example exhibits an exponential increase inthe elongational viscosity as a function of time, characteristic ofstructural hardening under stress (melt strength).

The dynamic viscosity is determined by means of a fixed-strainrheogoniometer sold by Rheometrics under the name ARES (AdvancedRheological Expansion System). The measurements are carried out on thespecimen which is placed between two parallel plates and subjected to astrain, the specimen, with a diameter of 25 mm and a thickness of 2 mm,being cut from a pressed plaque. The curve plotted in FIG. 2 (ARES plot)shows the variation at 170° C. in the dynamic viscosity expressed inPa.s as a function of the frequency expressed in rad/s.

The polymer produced in the example is characterized by an increase inthe dynamic viscosity at low frequencies.

Transmission electron microscopy is carried out by means of a ZEISS EM910 microscope. The examination was performed on ultramicrotomedsections approximately 90 nm in thickness.

FIG. 3 shows the photograph obtained by transmission electronmicroscopy. Ionic aggregates having a shape similar to that of a bunchof grapes may be seen in this figure, the size of the “bunch” being ofthe order of 100 to 200 nm and the size of the “grapes” being of theorder of 10 nm.

Analyses carried out by X-ray microanalysis using a LINK eXL II systemattached to the ZEISS EM 910 microscope clearly demonstrate the presenceof zinc in the ionic aggregates, particularly in the “grapes” making upthe “bunch”.

What is claimed is:
 1. A process for producing a polyolefin, comprisingthe following steps: a) grafting acid groups onto a polyolefin by meansof a graftable monomer bearing at least one functional group chosen fromthe group consisting of a carbonyl and an acid anhydride, optionally inthe presence of another graftable monomer bearing a vinyl unsaturatedgroup and, optionally one or more aromatic rings; b) purifying theproduct of step a) by removing at least a portion of the graftablemonomer bearing at least one functional group chosen from the groupconsisting of a carbonyl and an acid anhydride that has not grafted withthe polyolefin to produce a purified product; c) neutralizing the acidgroups of the purified product by reacting with zinc acetate, thegrafting step a) and the step c) being carried out in an extruder. 2.The process according to claim 1, wherein purifying is carried out byremoval of at least a portion of the graftable monomer with acetone, byhot-air stripping, by steam stripping, by stripping with an inert gas orby venting.
 3. The process according to claim 1, wherein the polyolefinis chosen from the group consisting of propylene polymers and ethylenepolymers.
 4. The process according to claim 1, wherein the graftablemonomer bearing at least one functional group is chosen from the groupconsisting of a carbonyl and an acid anhydride is maleic anhydride. 5.The process according to claim 1, wherein the graftable monomer bearingat least one functional group chosen from the group consisting of acarbonyl and an acid anhydride is added in an amount of from 0.01 to 10%by weight with respect to the polyolefin.
 6. The process according toclaim 1, wherein the process is carried out in the absence of agraftable monomer bearing a vinyl unsaturated group and, optionally oneor more aromatic rings.
 7. The process according to claim 1, whereingrafting of the acid group is carried out in the presence of a radicalgenerator.
 8. The process according to claim 1, wherein the zinc acetateis added in an amount of from 0.1 to 1000 meq./kg of polyolefin graftedwith acid groups.
 9. The process according to claim 1, furthercomprising the step of purifying d), said purifying d) comprisingremoving a product arising from neutralizing the grafted acid groups,wherein purifying d) follows neutralizing c).
 10. The process accordingto claim 9, wherein purifying d) is carried out by steam stripping, byhot-air stripping or by venting.
 11. The process according to claim 1,wherein the extruder is made of a corrosion-resistant alloy.
 12. Theprocess according to claim 1, wherein the process is carried out in acontinuous process comprising a single step for melting the polyolefin.13. The process according to claim 1, further comprising adding one ormore additives during the process.
 14. The process according to claim 1,further comprising adding at least one stabilizer during the process.15. The process according to claim 14, wherein the stabilizer is acompound comprising a sterically hindered phenol group, a phosphoruscompound or a mixture thereof.
 16. The process according to claim 15,wherein the stabilizer is a mixture of pentaerythrityltetrakis(3,5-di-tert-butyl-4-hydroxyphenylpropionate) andtris(2,4-di-tert-butylphenyl)phosphite.
 17. The process according toclaim 15, wherein the stabilizer is1,3,5-trimethyl-2,4,6-tris-(3,5-tert-butyl-4-hydroxybenzyl)benzene. 18.A polyolefin produced by the process of claim 1, comprising a pluralityof partially neutralized acid groups, said polyolefin having a melt flowindex, measured at 230° C. under a load of 2.16 kg for propylenepolymers according to ASTM D 1238 Standard and at 190° C. under a loadof 5 kg for ethylene polymers according to ISO 1133 Standard, of from0.001 to 1000 dg/min. and an improved melt strength characterized by anexponential increase in the elongational viscosity as a function of timeand by an increase in the dynamic viscosity at low shear frequencies.19. The polyolefin according to claim 18, comprising ionic aggregateshaving a shape similar to a bunch of grapes, the size of said bunchbeing between 10 and 500 nm and the size of a grape being less than 50nm.
 20. A process comprising extrusion foaming, thermoforming or blowmolding the polyolefin claimed in claim
 18. 21. A process comprisingcompatibilizing, sealing or adhering wherein the polyolefin claimed inclaim 18 is the compatibilizer, sealant or adhesive.
 22. An extrusionfoaming process comprising extrusion foaming a material, wherein saidextrusion foaming process is carried out consecutively to the polyolefinproduction process claimed in claim 1.